In such a machine a thread is reciprocated across a needle bed at a speed equaling that of a cam (or set of cams) serving to raise and lower a group of active latch needles successively engaging the thread to form a row of loops known as a course, the number of active needles determining the width of the resulting fabric. The thread or yarn presented to the latch needles trails the tip of a thread guide which is displaced by a transport mechanism synchronized with the cam drive. If a color change (or, for that matter, any other kind of substitution of one thread for another) is to be carried out within a course, a second thread guide must be activated in place of the first one; the change from one thread guide to another, however, creates problems inasmuch as the two trailing thread portions overlap for a considerable distance so that the two threads are simultaneously present in a succession of loops forming a transition zone between two differently colored areas of the pattern. A sharp boundary between these areas is therefore difficult to achieve, especially in high-speed machines in which the operator cannot manually intervene to prevent the simultaneous laying-in of two threads.